Invasive Therapy: 19 Minimally Invasive Therapy with Tooth-Colored Direct Restorative Materials
Minimally invasive caries therapy with composites has revolutionized restorative dentistry.1–3 With the help of composites, it is possible to restore teeth while maintaining a maximum amount of sound enamel and dentin, so Black′s rule “extension for prevention” is no longer considered generally valid.4–8 The fundamental prerequisite for the use of composites is effective adhesion to enamel and dentin (Chapter 14).9–12 Other materials besides composites are considered of secondary importance in this chapter. Today, composites and compomers are used to an almost equal degree,13–15 whereas glass-ionomer cements tend to be used for semipermanent restorations.16
In the posterior region, amalgams are still the reference against which other materials are compared, owing to their durability, functionality, and role as a basic restoration material17; however, amalgam restorations have been superseded for quite a while in dental practices with tooth-colored composite restorations. Attention remains focused on such critical factors as abrasion, secondary caries, and fractures, since these have been the weak areas for decades in posterior composite restorations.2,11,17–20 Every new development must address these issues. In general, function is the primary consideration in the posterior region, that is, clinical performance, with the cofactors of approximal contact, protection of the pulp, and resistance to abrasion and fractures.5,6,21,22 Esthetics are of secondary importance in this region, since it is mostly not necessary to render posterior fillings invisible to satisfy the patient.
In the anterior region, esthetically perfect restorations are generally recommended and explicitly desired by patients. Cutting-edge biomimetic composites are a welcome alternative to conventional, universal composites.
Function is paramount in posterior restorations, whereas esthetics is generally the chief consideration for anterior teeth.
In general, a minimally invasive approach involves more than just defect-oriented preparation. If the access in enamel to the cavity is too small to allow for proper application of the adhesive this would jeopardize the whole procedure. The compromised adhesion might result in an untimely renewal of the restoration due to adjacent caries, finally resulting in a rather large restoration. Furthermore, the major advantage of composite materials is that they can be repaired and corrected. Since removing all the composite is associated with the hazard of the iatrogenic removal of sound dentin and enamel and exposure of the pulp repair of partially insufficient restorations is a good alternative.23 The three factors for successful minimally invasive therapy are therefore defect-oriented preparation, permanently adhesive restorations, and reparability.
This chapter addresses the following topics:
Indications for restorative therapy
Groups of materials used for direct resin restorations
The choice of materials for different indications
The rules of minimally invasive preparations
Clinical experience with minimally invasive restorative therapy
Corrections or repairs of existing restorations
Indications for Restorative Therapy
The decision for invasive therapy of approximal caries in posterior teeth is made on the basis of a visual-tactile assessment and radiographic examination of the caries (Chapters 5, 6, 9, and 20). Restoration of an approximal carious tooth surface is indicated when it is cavitated and/or the caries extends radiographically into the middle third of the dentin ( Fig. 19.1 ),24–26 since noninvasive or microinvasive interventions are no longer sufficient to arrest the progress of the caries.
The appearance of anterior teeth is generally very important to an individual′s self-image and social acceptance. Carious hard tissues, tooth discoloration, existing restorations, and insufficient margins are generally perceived very quickly by others. Consequently, therapy chiefly needs to focus on restoring the natural appearance of the upper anterior teeth in particular. If lesions are diagnosed and treated when they are small, their treatment (e.g. repair) is easier, quicker, and more successful. In addition, restorations may need to be replaced when the discoloration of the surface or margins of existing composite fillings impair the esthetic appearance of the anterior teeth. Such situations can also be remedied by correcting the restoration.
Manifestations and Causes
Cervical defects have many different manifestations. Some have shell-like configurations, others have the shape of wedges with sharp edges, others are caused by caries, and others manifest different levels of enamel and dentin loss due to wear. All of these lesions can lead to increased sensitivity or esthetic impairment.
Since people are increasingly retaining their teeth, even in advanced age, the cervical area is increasingly becoming a particular problem area. This represents a challenge for dental therapy, since cervical restorations have a much lower survival rate than restorations in other areas.27,28
Although the appearances of cervical lesions can vary widely, they frequently have the same result: increased sensitivity. As the gingiva recedes, dentin is exposed. The superficial cementum layer wears off relatively quickly, exposing dentin tubules which can result in hypersensitivity. Morphological studies have revealed that open tubules are more frequently found in areas of increased dentin sensitivity.29 Within the tubules, the dentin liquor can be forced to move by osmotic, mechanical, or thermal stimuli. This explanation corresponds to the hydrodynamic theory of Brännström,30 according to which A-delta fibers are stimulated, which explains the stabbing, sharp pain experienced by the patient. The goal of treating sensitive cervical areas is to seal the tubules and thereby prevent the movement of the dentin liquor by external stimuli.
Dentin hypersensitivity can also arise from the removal of the enamel caused by the (cervical) erosion of the tooth surfaces, for example,31 which generally also includes a certain amount of mechanical abrasion (Chapters 2 and 3). In combination with occlusal overloading, mechanical abrasion can lead to wedge-shaped defects with sharp edges without requiring the effect of acid. The etiological factors are considered to be horizontal brushing and a strong eccentric load on the tooth. It has been proposed that the tooth bends under the functional load which can contribute to the cervical defect. As photoelastical images reveal, the development of stress is the greatest at the interface between the enamel and cementum when the tooth is under a functional load. The deeper the wedge-shaped defect, the stronger the bending of the tooth.32,33
Cervical defects in the form of lost dentin and enamel can assume different shapes such as craters or sharp-edged defects. Frequently, patients complain of elevated and sometimes painful sensitivity in the exposed cervical dentin.
There are therefore many reasons apart from caries why cervical defects develop which must each be considered when treating the patient, so that the progress of the lesion can be halted and proper restorative material can be selected ( Fig. 19.2 ).
Early identification of cervical defects is important so that the patient can be offered early enough instruction regarding appropriate oral hygiene.
Technical Limits to Direct Restorative Materials
Long-lasting reconstructions can be inserted that appear almost natural by combining different opacities and transparencies ( Fig. 19.3 ). The appropriateness of laboratory-made veneers increases with the extent of the defect in the labial surface. The extension of veneers passes smoothly to ceramic partial or full crowns.
In the posterior region, direct resin restorations tend to be preferred over indirect restorations with, for example, ceramics. Minimally invasive initial restorations of caries lesions are the ideal indication for the direct use of composites; however, nowadays the routine work in restorative dentistry mainly includes the replacement of insufficient restorations, which become larger each time, in particular when caries lesions have developed adjacent to the restorations. There are three potential problems or risks associated with direct composite restorations:
Shrinkage during polymerization. The larger the cavity, the greater the problems associated with the shrinkage of the composite during polymerization.9,20,34,35 The tension exerted on the enamel structures increases as the support offered by the dentin decreases, which leads to a greater number of cracks that can weaken the dental hard tissues and endanger the overall integrity of the bond to the tooth ( Fig. 19.4 ). Horizontal fracture lines are particularly problematic, whereas vertical enamel cracks occur in practically all adhesively restored teeth, especially with greater drying.4,15,20
Abrasion behavior. As long as the occlusion has a certain amount of support in the intact enamel, the occlusal composite will be subject to less load than if there is an extensive composite reconstruction of the masticatory surface.9,20,36 Whereas minimally invasive composite restorations are protected from being abraded by the enamel components, partial or complete reconstructions of the masticatory surface suffer significantly more abrasion.3,8,15,21 This does not cause the failure of the restoration; however, the anatomical shape can be slowly lost over time ( Fig. 19.5 ).
Time and effort for the dentist. The shrinkage of the composite is less in small, intracoronal reconstructions with a sufficient amount of residual dentin and enamel; in addition, the amount of effort required by the dentist is limited since the remaining hard tissues in the occlusal area makes it significantly easier to model the composite material and reconstruct the masticatory surfaces.15 However, if entire occlusal surfaces and cusps need to be reconstructed, the amount of effort required by the dentist is significant.37 Since occlusion cannot be checked during modeling when using the contamination-sensitive adhesive technique, a great deal of practice and time are required. In addition, there is a certain risk of mislocating in the tip of the cusps as well as important anatomical structures such as central cusp slopes and marginal ridges.
Polymerization shrinkage, abrasion, and sensitivity to the technique are the major limitations of resin restorative materials.
Materials for Tooth-colored Direct Restorative Therapy
Various materials can be used to create tooth-colored resin restorations. In the developmental history of restorations, it was first attempted to adapt classic cements to the color of teeth. From this group of cements, only glass-ionomer cement remains relevant in practice, which is easy to process since it independently adheres to the dentin and enamel. In contrast, composites can achieve a much more esthetically attractive result. Their translucence approaches the natural appearance of teeth. However, when this group of materials is used, the cavity must be pretreated with adhesive systems, which is relatively time-consuming.
For this reason, there have been several efforts to modify conventional glass-ionomer cements. One of these modifications is resin-modified glass-ionomer cements, another is compomers.
Glass-ionomer cements are made of a mixture of powdered glass and polyalkene acids. They are also termed polyacrylic acids and typically contain carboxylate groups. The primary advantage of this group of materials is their release of fluoride, the clinical efficacy of which is a subject of controversy. In comparison to composites, the cavity is relatively easy to pretreat. Glass-ionomer cements adhere chemically to enamel and dentin trough the ion bonds of the carboxylate groups. However, the esthetic appearance of glass ionomer cements is less satisfactory as they are usually highly opaque. Furthermore, materials of this group are less resistant to abrasion and are very sensitive to drying and contamination with saliva or blood during the setting phase.38
Glass ionomer cements have the advantage of chemically bonding to the dental hard tissues.
Products made of composite materials are the most widespread and frequently used for therapy with tooth-colored resin restorative materials. These consist of monomer systems that possess at least two methacrylate groups per residue and harden by polymerization. Embedded in this matrix are fillers of different types and sizes that can form chemical bonds with the monomers by means of silanization to improve stability. A categorization of these composites by fillers is shown in Fig. 19.6 . Developments in recent years have yielded commercially available composite fillers that have satisfactory physical properties and are generally suitable for use in dental practice. Composite fillers have been continuously refined over the years, since they are responsible for nearly every clinically relevant factor of a system. For example, the composition of the filler and its particle size determine important properties such as the stress arising from polymerization shrinkage, abrasion resistance, fracture toughness, radiopacity, polishability, and adhesion and hence handling by the user.
A monomer matrix
Fillers of various types and sizes
Silanes which bind the fillers to the matrix
Fillers are now so advanced that any significant reduction of polymerization shrinkage can only be achieved by new approaches in matrix chemistry: The recently introduced silorans (Filtek Silorane by 3 MEspe, Seefeld, Germany) represent a step in this direction with their use of ring-opening systems. They offer a product with shrinkage below 1%.39 With classic composites, a linear shrinkage of about 1.5% appears to be a bottom threshold that cannot be reduced any further through innovations in fillers.
Abrasion resistance which can significantly influence the longevity of a restoration that is subject to mastication has been enhanced to a satisfactory clinical level.40 This also implies that the classical division of indications for the anterior and posterior composites has become outmoded: All commercially available systems are quite capable of satisfying the high physical requirements of abrasion resistance and bending resistance in the posterior region. Practically the only identifiable differences are in polishability.41
Since the material properties of the composites have apparently been developed to a level that is difficult to surpass, classifying composites by filler particles ( Fig. 19.6 ) is important to understand historical development, but no longer helps the practitioner to choose a system for his or her practice. An alternative classification of composite materials according to translucence and opacity can therefore help dentists select the right composite from the broad spectrum of products. A distinction can be made between product lines that offer a single transparency or opacity and those that offer up to three. There are only relatively few composites that offer only one translucency. These provide basic restorations without esthetic considerations, and go so far as to state that they are only available in a single color.
There are two basic types of modern product lines that focus on esthetics. Whereas some manufacturers retain and refine the approach of three opacities/translucencies, other companies focus on the natural anatomy of the tooth. Here only dentin and enamel composites for highly esthetic restorations are available, composites that are consequently limited to two translucencies. These two different approaches determine the layering technique for the specific composites. In general, a distinction can be drawn between the two-layer and three-layer technique corresponding to the number of translucencies ( Fig. 19.7 ).42
Composites can therefore be used to create restorations esthetically adapted to the available hard tissue, so that the defects are no longer visible to the naked eye.
In terms of the appearance, the categorization of composite products according to opacity and translucency makes more sense than categorization by fillers.
Resin-Modified Glass-Ionomer Cements
With resin-modified glass-ionomer cements, methacrylate groups were incorporated into the polyacrylic acid molecule. This rendered the material light-curable, and adding hydrophilic monomers elevated the fracture toughness of the products in comparison to classic glass-ionomer cements.43
Products belonging to this group need to be mixed. In addition to the polymerization of the hydrophilic monomers, the acid/base reaction of polyacrylic acid with the glasses requires water as a medium. Resin-modified glass-ionomer cements therefore have a longer setting time than classic glass-ionomer cements, however, the curing reaction starts immediately on exposure to light. Their curing time is therefore less than that of classic glass-ionomer cements.
Resin-modified glass-ionomer cements contain monomers in addition to the components of the classic glass-ionomer cements.
The name of this group of materials suggests that it consists of a mixture of glass-ionomer cements and composite. These are essentially polyacrylate-modified composites, since carboxylate groups are added to the diacrylate monomer in this group of materials.44 The subsequent absorption of water causes the reaction with the glasses (fillers). Release of fluoride, although to a much lesser extent than with glass-ionomer cements, is measurable. Nonetheless, there exists some uncertainty as to whether the release of fluoride is clinically relevant.45
In any case, products in this group of materials have a higher abrasion resistance than glass-ionomer cements.46 On the other hand, compomers do not independently adhere to the dentin and enamel, and they must be used together with an adhesive system to achieve gap-free margins.
Compomers are carboxylate-modified composites and need to be used together with an adhesive system.
Choosing Materials for Different Indications
Whereas glass-ionomer cements are generally used for semipermanent restorations,16 composites allow permanent restorations to be inserted.5–7, 20,37 The range of indications for direct composites extends from minimally invasive restorations to the replacement of cusps.3,8,22,37
Esthetically satisfying restorations in the anterior region can be reliably achieved with composite materials. Clinical studies have demonstrated, however, that compomers can produce satisfactory results for a period of more than two years.48,49 Over the long term, the surface gloss and color stability of composite materials are better.50